The detection of electromagnetic radiation of various frequencies has a number of important applications. Because of the well understood relationship between light energy and various physical, electrical and chemical phenomena, many such events can be identified by the frequency (or correspondingly by the wavelength) of the electromagnetic radiation that they emit or absorb. As typical examples, movements of atoms within molecules tend to emit radiation in the infrared portion of the spectrum. Transitions within the nucleus of atoms tend to emit radiation in the X-ray frequencies. Rotational movements of molecules tend to emit in the microwave frequencies. Electronic transitions between energy levels in molecules and atoms tend to emit and absorb in the visible portion of the spectrum and, of particular interest to the present invention, in the ultraviolet portion of the spectrum.
As brief background, the characteristics of light energy are most frequently expressed as wavelength (the length of one complete periodic wave cycle) and frequency (how many complete cycles occur in a given time period). It is well understood that wavelength and frequency are inversely proportional to one another and related to the speed of light by the relationship: EQU Wavelength X Frequency=Speed of Light
or, expressed in the usual symbols: EQU .lambda..nu.=c
Longer wavelengths thus correspond to smaller frequencies and shorter wavelengths correspond to higher frequencies. In turn, the frequency of light emitted or absorbed as a result of a particular event is directly proportioned to the energy associated with the event. This is conveniently expressed as: EQU E=h.nu.
in which E represents energy, .nu. represents the frequency, and h represents Plank's constant.
Detection of ultraviolet radiation is of particular interest because such radiation is typically produced by combustion phenomena. Thus, ultraviolet radiation detectors can discriminate between the presence of combustion and the presence of heat in a manner in which infrared radiation detectors, which tend to indicate the presence of heat regardless of the presence of combustion, cannot.
As would be expected, the ability to detect ultraviolet radiation has a number of significant commercial and military applications. As one commercial example, some type of ultraviolet detector is a required component in any system using ultraviolet light to transmit information. Because ultraviolet light has a higher frequency than visible light, it can carry more information. Thus, the use of ultraviolet detectors in fiber optical information transmission systems and couplings is of particular interest.
In military applications, the detection of the presence of combustion, particularly in a highly sensitive manner, has obvious advantages such as identifying the presence of aircraft, missiles, or other objects propelled by characteristic combustion processes. In such military applications, a further desirable advantage is for the detector to be "solar blind." In other words, because the ultraviolet and visible portions of the electromagnetic spectrum are adjacent one another, ordinary visible sunlight can tend to cause a response in an ultraviolet detector down to approximately 290 nanometers (nm) which can either blind the detector to the presence of ultraviolet radiation or greatly reduce its sensitivity.
One way of detecting radiation, regardless of frequency or wavelength is to expose a semiconductor ("solid state") material to the radiation and note the response in the semiconductor material. Semiconductors are particularly advantageous for detection systems for a number of well known reasons. These typically include their small size, stability, reliability and lower cost. Solid state photodetectors fall into separate categories which generally include photoconductors, photodiodes and phototransistors. A photoconductor is a portion of semiconductor material with ohmic contacts affixed to opposite ends. When struck by incident light, carriers are generated therein which result in an increase in the material's conductivity. A photodiode is a p-n junction device with an associated depletion region. The depletion region's electric field separates photogenerated electron-hole pairs, the movement of which generates a measurable current. A phototransistor is similar to a conventional bipolar transistor but has a large base-collector junction as the light collecting element. Of these devices, the photodiode is one of the most commonly used photodetection device for visible and ultraviolet light.